1. Field of the Invention
The present invention relates to a fuel cell stack formed by stacking a first electrolyte assembly and a second electrolyte electrode assembly and separators alternately in a stacking direction. Each of the first electrolyte electrode assembly and the second electrolyte electrode assembly comprises a pair of electrodes and an electrolyte interposed between the electrodes. One reactant gas passage as a passage of one reactant gas and another reactant passage as a passage of another reactant gas extend through the separators in the stacking direction.
2. Description of the Related Art
For example, a polymer electrolyte fuel cell employs a membrane electrode assembly which includes an anode, a cathode, and an electrolyte membrane (electrolyte) interposed between the anode and the cathode. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly is sandwiched between a pair of separators. The membrane electrode assembly and the separators make up a power generation cell for generating electricity. In practical use, generally, a predetermined number of power generation cells are stacked together to form a fuel cell stack.
In the fuel cell stack, a fuel gas flow field (reactant gas flow field) for supplying a fuel gas (reactant gas) to the anode and an oxygen-containing gas flow field (reactant gas flow field) for supplying an oxygen-containing gas (reactant gas) to the cathode are provided. Further, as necessary, a coolant flow field is provided between the separators for supplying a coolant along the surfaces of the separators.
In general, the fuel cells adopt, so-called, internal manifold structure in which fluid supply passages and fluid discharge passages extend through the separators in the stacking direction. The fluids, i.e., the fuel gas, the oxygen-containing gas, and the coolant are supplied from the respective fluid supply passages to the fuel gas flow field, the oxygen-containing gas flow field, and the coolant flow field, and then, discharged into the respective fluid discharge passages.
In order to perform the power generation efficiently, the fuel gas and the oxygen-containing gas need to be suitably supplied to the entire power generation surfaces. Each of the fuel gas flow field and the oxygen-containing gas flow field has a large number of flow grooves over the entire power generation surface. The opening area of the fluid supply passage is considerably small in comparison with that of the fluid grooves. In the structure, it is extremely difficult to uniformly supply the fuel gas and the oxygen-containing gas from the fluid supply passages to the respective flow grooves.
In this regard, a separator for a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2003-77497 is known. As shown in FIG. 17, the separator 1 has ridges (protrusions) 2 to 6 on a surface facing an anode (not shown). The ridge 2 is formed along the edge of the separator 1, and positioned outside the fuel gas inlet 7. The fuel gas flows through an area hemmed by the ridge 2.
An inlet section 8 as part of a serpentine fuel gas flow field is provided on a surface of the separator 1. Square protrusions 3 are provided at equal intervals laterally and longitudinally to form flow grooves 9 in a grid pattern as a whole. The inlet section 8 includes an expansion 8a having a width larger than that of the flow field at the fuel gas inlet 7.
Three strip-shaped ridges 4, 5, 6 are formed in the inlet section 8. The ridges 4, 5, 6 are formed on the upstream side, adjacent to the fuel gas inlet 7, and extend in parallel toward the downstream side. Then the ridges 4, 5 are turned upwardly in a zigzag pattern, and the ridge 6 is turned downwardly in a zigzag pattern. According to the disclosed structure, the fuel gas supplied from the fuel gas inlet 7 to the inlet section 8 flows uniformly over the entire inlet section 8 by the guidance of the ridges 4 to 6.
However, in the conventional technique, the inlet section 8 is provided on one surface of the separator 1, and another inlet section 8 is provided on the other surface of the separator 1. Therefore, in order to reduce the thickness of the separator 1, since each of the inlet sections 8 should be short in the height direction, the flow grooves 9 may not have sufficient depth, and thus, the pressure loss is large. In particular, in the case of adopting metal separators, since the flow fields are formed on both front and back surfaces of the separator (the grooves on one surface form the ridges (protrusions) on the other surface), the separator cannot be designed freely.